Opportunistic routing with network coding (NCOR) has emerged as a promising approach to improve both the throughput and reliability in lossy wireless multi-hop networks. This new routing paradigm, which is based on the multi-user diversity advantage of wireless broadcast links and the erasure coding property of random linear network coding, brings opportunities and challenges for broadcast MAC designs. This paper carries out a study on the opportunistic broadcast channel access problem with the intent on appealing to the optimal stopping theory, the study proposes an access strategy to achieve the maximal average effective rate, which is a trade-off between the access delay and the instantaneous delivery ability. By extending IEEE 802.11 DCF, the study further presents a MAC protocol O-BCast to execute the proposed strategy. Simulation results show that O-BCast achieves notable improvement of NCOR’s end-to-end throughput and is adaptive to various network loads.